It is known to fold paper or cardboard blanks into bowl-shaped containers, which can be filled with a powder or baked foodstuffs and then covered by a lid. These bowl-shaped containers, such as the one disclosed in German Offenlegungsscrift No. 28 19 708, can include a paper and/or cardboard substrate, which is printed on with ink in an appealing manner and coated, on an exterior surface, with a layer of polypropylene. An interior surface of the substrate is coated with a layer of polyethylene, which acts as a laminating agent for a piece of aluminum foil laminated to the polyethylene. After the aluminum foil is laminated to the polyethylene, a separating agent is applied by a lacquering machine, so that the baked goods or other contents of the container can be easily removed therefrom.
The choice of coatings for the cardboard substrate is determined primarily by the intended use for the container. If, for example, the container is filled with a powdered material to be baked in an oven after removal of the lid, then the coating must withstand temperatures of about 200.degree. C., because such baking processes are normally carried out at a temperature of about 175.degree. C. for approximately forty-five minutes. If the composition of the laminate is the same as the one described in German Offenlegungsscrift No. 28 19 708, then there are problems in that leaks can be formed in the vicinity of the corners of the container, due to difficulities in sealing the aluminum foil, especially if the aluminum foil has been coated with the separating agent.
Inasmuch as the temperature limit for the baking process is determined by the synthetic materials employed as the coatings, there was a need for different coatings which would facilitate heat-sealing and withstand the temperatures experienced during the baking process. For example, the temperature limit lies in a range of from about 100.degree. C. to about 105.degree. C. for polyethylene, in a range of from about 145.degree. C. to about 150.degree. C. for polypropylene, and in a range of from about 220.degree. C. to about 225.degree. C. for polyester, such as PBTP or PETP.
The polyesters are the most difficult synthetic materials to heat-seal. This heat-sealing difficulty results from the relatively short temperature range between the liquid and solid states of the polyesters.
In the past, heat for heat-sealing the blanks has been transferred through the cardboard substrate to the synthetic coating. However, transferring heat through the cardboard substrate can burn the cardboard, thereby creating brown spots which detrimentally affect the appearance of the completed container. Also, the printing ink applied to the cardboard could be damaged by being heated too much.
Synthetic coatings disposed on cardboard substrates have, in the past, also been activated, i.e., softened, plasticized, or liquidized, by hot air applied directly to the synthetic coatings. Activation of a synthetic coating by the direct application of heat thereto becomes difficult when the synthetic coating is a polyester, because the intense heat required to activate the polyester causes residual water in the cardboard, which residual water constitutes about 6% or 7% of the volume or weight of the substrate, to evaporate and explode, thereby bursting the cardboard substrate. Also, care must be taken to apply sufficient heat to the synthetic coating to maintain the activation state of the synthetic materials during transportation of the blanks to a shaping station.